Download the zip in soopa's first post, follow his instructions. Open the "Run_AC3gain.bat" file in notepad and change the -b 384 to -b 640
Save the file then double click it.

I can't recall if Aften has 640K bitrate, but it might. Give it a try and if 640 doesn't work, try 448. I've never seen a commercial DVD with AC3 audio over 448. DTS is a completely different story, however.